Method and apparatus for the alignment of a substrate

ABSTRACT

Method comprises a process for obtaining a photoelectric signal with a waveform pair of extremal values at respective positions corresponding to a pair of edge portions of an alignment mark by photoelectrically detecting the reflected light from the alignment mark on a substrate; a first determination process for determining the position of the alignment mark on the basis of a pair of slope portions existing inside the pair of extremal values of the photoelectric signal waveform; a second determination process for determining the position of the alignment mark on the basis of the pair of slope portions existing outside a pair of extremal values of the photoelectric signal waveform; a third determination process for determining the position of the alignment mark on the basis of both a pair of slope portions existing inside said pair of extremal values of the photoelectric signal waveform and a pair of slope portions existing outside; and a process for selecting any one of the first determination process, second determination process, or third determination process in accordance with the objective alignment accuracy of the substrate.

This is a division of application Ser. No. 08/136,991 filed Oct. 18,1993 (abandoned), which was continued as application Ser. No. 08/390,285filed Feb. 15, 1995, now Pat. No. 5,493,403 issued Feb. 20, 1996, andwhich is a continuation of application Ser. No. 07/722,157 filed Jun.27, 1991 (abandoned).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and apparatus for performingalignment by photoelectrically detecting the alignment mark formed on asemiconductor wafer, a plate for liquid crystal display, or the like.

2. Related Background Art

Traditionally, there has been generally used in positioning the wafer,plate, or the like (in alignment), a method of photoelectricallydetecting the alignment mark formed at a predetermined position on asubstrate through the objective lens of a microscope.

The photoelectric detection method is roughly divided into two kinds,i.e., the light beam scanning method in which the mark is relativelyscanned by the spot of a laser beam or the like, and the scattering raysof light or refraction light generated by the mark is received by aphotomultiplier, photodiode, or the like; and the method which utilizesimage signals obtained by a television camera (Vidicon tube or CCD)picking up the enlarged image of a mark evenly illuminated.

In either case, the waveform of such photosignal is processed to obtainthe central position of the mark.

Although the light beam method and the pick-up method are completelydifferent in its structures of scanning system, these two are consideredhere as an electrical-optical scanner, respectively (hereinafterreferred to as E.O.S.).

Among such E.O.S.'s, there is known a technique as a method of detectingthe mark position by carrying the wafer stage one-dimensionally againstthe laser beam spot such as disclosed in U.S. Pat. No. 4,655,598, U.S.Pat. No. 4,677,301, and U.S. Pat. No. 4,702,606.

Also, there is known a technique as a method of detecting the markposition within the region of the one-dimensional scanning subsequent tothe positioning of the wafer stage by a designed value such as disclosedin U.S. Pat. No. 4,390,279, and U.S. Pat. No. 4,566,795.

Also, as an E.O.S. using a pick-up method, there is known a techniquesuch as disclosed in U.S. Pat. No. 4,402,596, U.S. Pat. No. 4,679,942,and U.S. Pat. No. 4,860,374.

In these conventional techniques, a monochromatic light is used as ascanning beam or mark illuminating light mainly for two reasons givenbelow.

1. In a projection type aligner (stepper), a single-wavelengthilluminating light or laser beam is used in order to avoid any largechromic aberration for the type which detects the wafer mark through theprojecting optical system.

2. A monochromatic laser beam is used to enable a fine spot convergenceof the beam for performing a high-luminance and high-resolutiondetection. When a monochromatic illuminating light (or beam) is used asset forth above, the obtainable S/N ratio is comparatively large.However, there appears an interference phenomenon due to themonochromaticity because a photoresist layer of 0.5 μm-2 μm thick isusually formed all over the wafer surface, and this often results in adetection error when the mark position is detected or makes an objectiveimage unclear.

Therefore, in order to reduce the interference phenomenon caused by theresist, there has been proposed in recent years the application ofmulti-wavelength or wide band to the illuminating light.

For example, an illuminating light is produced by a halogen lamp or thelike for the pick-up type E.O.S., and if the wavelength bandwidththereof is set for approximately 300 nm (with the exception of thephotosensitive region for the resist), the coherence of the raysthemselves reflected from the resist surface and the wafer surfacealmost disappears; thus making it possible to carry out the detection ona clear image. Therefore, in the pick-up method, if only a white (wideband) illuminating light is used with an achromatic image-formationoptical system, an extremely precise alignment sensor which is notaffected by the resist is obtainable.

As the above describes, with the application of a polychromatic or whiteilluminating light, it becomes possible to restrict the generation ofthe interference fringe for an excellent image detection. Then, theextremely small factors causing errors which have passed unnoticed cometo the fore.

In other words, since the staged structure of the alignment mark isclearly detected, a slight difference in the profiles of the mark edgesbecomes capable of affecting the precision of the detection oralignment.

Traditionally, various algorithms have been worked out for image signalprocessings. However, none of them have ever taken into considerationsuch slight changes in the mark edge profiles, and there hasautomatically been a limit for an overall improvement of the alignmentaccuracy.

SUMMARY OP THE INVENTION

The present invention is designed in consideration of such problems asmentioned above, and the object thereof is to improve the alignmentaccuracy.

The present invention relates to a method and apparatus forphotoelectrically detecting the optical information originating from thealignment mark on a substrate such as a wafer by an E.O.S. such as atelevision camera and scanning laser to determine the position of thealignment mark by processing the time series photoelectric signal (imagesignal) which changes its intensity with respect to the relativescanning direction of the alignment mark.

The present invention is designed to provide the processes given below:

a process to obtain a photoelectric signal waveform which has extremalvalue at respective positions of a pair of mark edge portions whichdefine the mark width;

a first determination process to determine the mark position based on apair of slope portions existing inside the two extremal values in thephotoelectric waveform;

a second determination process to determine the mark position based on apair of slope portions existing outside the two extremal values;

a third determination process to determine the mark position based onthe slope portions existing both inside and outside the extremal values;and

a process to select any one of the first determination process, seconddetermination process, or third determination process in accordance withthe target alignment precision for the substrate.

Fundamentally, the waveform processings of the signal shown in FIG. 2 isperformed in the present invention.

FIG. 2(a) shows the cross-sectional structure of a convex mark MK formedon a wafer W with the resist layer PR covered evenly over the surfacethereof.

FIG. 2(b) shows the waveform of video signal VS of the image of the markMK picked up by a television camera along the scanning line across theedges E1 and E2 of the mark MK. This video signal VS represents thebottom waveforms BW1 and BW2 which become minimal values at thepositions of both edges E1 and E2 of the mark MK. The waveform levelbetween the bottom waveform portion BW1 and BW2 varies by thereflectance of the mark MK itself, and the waveform level on the leftside of the bottom waveform portion BW1 and the waveform level on theright side of the bottom waveform portion BW2 vary by the reflectance ofthe wafer substrate.

FIG. 2(c) is an enlarged representation of the two bottom waveformportions BW1 and BW2. The bottom waveform portion BW1 has a down-slopeportion DSL1 which falls down to the bottom level BT1 as the scanningadvances, and an up-slope portion USL1 which rises from the bottom levelBT1. Likewise, the bottom waveform portion BW2 has a down-slope portionDSL2 which falls down to the bottom level BT2 and a up-slope portionUSL2 which rises from the bottom level BT2.

In the present invention, the central position of the mark MK withrespect to the scanning direction is determined by selectively using thebottom waveform portion BW1 and BW2, and the slope portions DSL1, USL1,DSL2, and USL2 corresponding respectively to both edges E1 and E2 of themark MK.

In each of the slope portions, those slope portions existing inside arethe up-slope portion USL1 and the down-slope portion DSL2, and thoseslope portions existing outside are the down-slope portion DSL1 and theup-slope portion USL2.

In practice, the process is executed to obtain the scanned position P1where the slope portion DSL1 coincides with the slice level S1 whichdivides between the peak value at the shoulder of the down-slope portionDSL1 and the bottom level BT1 in one bottom waveform portion BW1 by apredetermined ratio (50%, for example), and the scanned position P2where the slope portion USL1 coincides with the slice level S2 whichdivides between the peak value at the shoulder portion of the up-slopeportion USL1 and the bottom level BT1 by a predetermined ratio.

Likewise, for the other waveform portion BW2, the process is executed todetermine the position P3 obtained by comparing the down-slope portionDSL2 with the slice level S3 and the position P4 obtained by comparingthe up-slope portion USL2 with the slice level S4.

Therefore, the computation of the central position Pm of the mark MK isfundamentally performed in accordance with any one of the threeequations given below.

    Pm=(P2+P3)/2                                               (1)

    Pm=(P1+P4)/2                                               (2)

    Pm=(P1+P2+P3+P4)/4                                         (3)

Here, the equation (1) is the basic expression to determine the insideslope; (2), the outside slope; and (3), both slopes.

Then, in the present invention, the alignment of a wafer is executed byselecting the determining equation which optimizes the precision wherebyan actual wafer is aligned, for example.

In this respect, while the present invention is equally applicable tothe alignment method employed for the synchroton orbital radiation (SOR)X-ray aligner, it is desirable to prepare an objective lens systemhaving an additional double-focusing element so that the marks of themask and wafer are detected simultaneously because in the X-rayaligning, the mark and wafer approach with a predetermined gap.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing the structure of a stepper suitablyimplementing the method according to an embodiment of the presentinvention;

FIGS. 2(a) to 2(c) are views showing the mark cross-section and signalwaveform illustrating the principle of the present invention;

FIG. 3 is a block diagram schematically showing the signal processingsystem of a CCD camera;

FIG. 4 is a plan view showing the shot array and mark arrangement on awafer;

FIG. 5 is a plan view showing the mark arrangement on an index plate;

FIGS. 6(a) and 6(b) are views showing the shape of wafer mark andsectional structure;

FIGS. 7(a) and 7(b) are views showing the arrangements of the index markand wafer mark at the time of alignment and the waveform of the videosignal from the CCD camera;

FIG. 8 is a flowchart illustrating the procedures on the alignmentprocess in accordance with the method according to an embodiment of thepresent invention;

FIGS. 9A(a) to 9A(c), 9B, 10(a) and 10(b), 11(a) and 11(b), 12A and 12Bare views showing the waveforms illustrating the states of signalwaveform data worked out in the course of process shown in FIG. 8;

FIGS. 13(a) to 13(c) are views showing the structure of asymmetric markand its signal waveform;

FIGS. 14(a), 14(b), 15(a) and 15(b) are views illustrating thedifference in the vernier configurations respectively;

FIG. 16 is a view illustrating the vernier reading;

FIG. 17 is a wafer plan view showing the state of the mark becomingasymmetric in the peripheral shots;

FIG. 18 is a view illustrating an example of TTR alignment sensor;

FIGS. 19(a) and 19(b) are views showing the sectional structure oflattice mark used for a coherent alignment method and its signalwaveform;

FIGS. 20A, 20B, and 20C are views showing the variations of the wafermark shape respectively;

FIG. 21 is a flowchart illustrating the procedure for selecting theoptimum mode by automatically collating the number of the wafer marksand that of edge bottom waveforms;

FIG. 22 is a view showing the waveform illustrating an example of thesignal waveform processing in the process shown in FIG. 21;

FIGS. 23(a) and 23(b) are views showing the mark structure illustratingthe split top phenomenon of the edge bottom wave and its signalwaveform;

FIG. 24 is a perspective view showing another embodiment of the waferalignment sensor shown in FIG. 1;

FIG. 25 is a plan view showing the mark arrangement on a conjugate indexplate suited for the system shown in FIG. 24;

FIGS. 26(a), 26(b), 27(a) and 27(b) are views showing the usage of theindex mark shown in FIG. 25 and the method of the signal processingrespectively;

FIG. 28 is a plan view showing the relationship between the globalalignment mark arrangement and the pick-up range at the time of searchalignment;

FIGS. 29(a) and 29(b) are views illustrating an example of the videowaveform when the wafer shown in FIG. 28 picked up; and

FIG. 30 is a plan view showing an example of the shot arrangement by thesample alignment with the E.G.A. method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

At first, in reference to FIG. 1, the structure of a stepper suited forimplementing the method according to an embodiment of the presentinvention will be described.

In FIG. 1, an image in the pattern region PA on a reticle R is projectedto form its image on the wafer W through a projection lens PL. The waferW is mounted on a stage ST movable in the directions X and Y by thestep-and-repeat method, and the coordinate position of the stage ST ismeasured by interferometers IFX and IFY. The reticle R is aligned to thestepper (the optical axis of the projection lens PL) by positioning thereticle alignment marks RM1 and RM2 provided at both sides of thepattern region PA for the reticle alignment microscopes RAS1 and RAS2.Also, in the region corresponding to the peripheral street lines of thepattern region PA, a mark (window) is formed for the die-by-diealignment, and each mark (window) is detected by TTR(through-the-reticle) alignment microscopes DAS1, DAS2, DAS3, and DAS4together with the wafer mark for the die-by-die attached to one shotregion on the wafer W.

Now, the method according to the present embodiment here is applicableto a wafer alignment sensor which detects only the mark on the wafer Wby the off-axis method. This wafer alignment sensor comprises a mirror10 arranged in the close vicinity immediately below the projection lensPL, an objective lens 12, a beam splitter 14, an image-formation lens16, a conjugate index plate 18, an image pick-up lens 20, and a CCDtwo-dimensional image pick-up element 22. Further, in order toilluminate the mark region on the wafer W, there is provided anilluminating optical system comprising the optical fiber 24 whichinduces light of wide band wavelength emitted from a halogen lamp,luminance polychromatic LED or the like, a condenser lens 26, anillumination range diaphragm 28, a lens system 30, and the beam splitter14 which has been mentioned earlier.

In the above-mentioned structure, the wafer W is arranged conjugatelywith the index plate 18 optically with respect to the synthetic systemof the objective lens 12 and image-formation lens 16, and the lightreceiving planes of the index plate 18 and CCD 22 are conjugatelyarranged with respect to the pick-up lens 20.

Therefore, the CCD 22 picks up the enlarged image of the mark on thewafer W and the enlarged image of the fixed (reference) mark on theindex plate 18 simultaneously. Also, the emission end of the fiber 24 ofthe illuminating optical system is relayed as a secondary light sourceimage to the pupil plane (aperture diaphragm position) between theobjective lens 12 and the lens system 30 to provide Kohler'sillumination for the wafer W. Furthermore, the range diaphragm 28 isconjugate with the wafer W by the synthetic system of the objective lens12 and the lens system 30, and the aperture image of the range diaphragm28 is conjugate with the wafer W. Accordingly, the aperture image of therange diaphragm 28 is projected onto the wafer W. In the presentembodiment, at least for each of the objective lens 12, image-formationlens 16, pick-up lens 20, the achromatic treatment is provided so as toprevent the deterioration of the image-formation characteristics due tochromatic aberration.

Also, in the apparatus according to the present embodiment, a referencemark FM is provided on the stage ST and is used for measuring thedistance (base line) between the projection point to the wafer W of theindex mark on the index plate 18 in the wafer alignment sensor and thereticle alignment marks RM1 and RM2 on the reticle R or the projectionpoint of the mark for the die-by-die.

Now, in reference to FIG. 3, the processing circuit for the videosignals from the CCD 22 shown in FIG. 1 will be described. The CCD 22 isa two-dimensional pick-up element with pixels arranged in the horizontalscanning direction and vertical scanning direction. In the CCD 22 of thepresent embodiment, however, the horizontal scanning direction isassumed to coincide with the direction crossing the mark edges on thewafer W.

Now, from the CCD 22, a composite video signal, which is a mixture ofthe horizontal synchronous signal and vertical synchronous signal, isobtained. This video signal is transferred to an analog-digitalconverter (ADC) 42 through the preparatory circuit 40 such as afrequency filter or AGC. Meanwhile, the video signal from the CCD 22 istransferred to a controlling circuit 44 including a synchronous signalseparator circuit, clock generator circuit and the like. Thiscontrolling circuit 44 outputs the clock signal SCL such that one clockpulse is generated per electrical scanning (reading scan) of one pixelin accordance with the horizontal synchronous signal from the CCD 22.This clock signal SCL is transferred to the comparator 46 which detectswhether or not the electrical scanning of the CCD 22 has covered thesampling region (scanning in the vertical direction of the horizontalscanning) in one frame, and to the address counter 48 which outputs theaddress value to the memory (RAM) 43 for storing the output data fromthe ADC 42. Therefore, in the RAM 43, the digital waveform data of thescanned portion designated by the predetermined horizontal scanning bythe CCD 22 is stored. The waveform data stored in the RAM 43 is read bythe processor 50 through the address bus A-BUS and data bus D-BUScontrolled by the process 50 to execute a given waveform processingoperation. To the address bus A-BUS and data bus D-BUS of the processor50, a stage controller 52 is connected to control the stage ST. With theinput of coordinating value of the interferometers IFX and IFY, thiscontroller 52 controls a driving motor 54 for the stage ST.

Next, in reference to FIG. 4, FIG. 5, and FIG. 6, the mark configurationand arrangement suited for the present embodiment will be described.

FIG. 4 shows the shot arrangement on the wafer W, and the projectedimage in the pattern region of the reticle R is aligned with each of theshot regions SA. Then, at the time of exposure, the center CC of each ofthe shot regions SA coincides with the center of the pattern region PAof the reticle R. The center lines rectangular at the center CC areparallel to the X axis and Y axis of the linear coordination regulatedby the interferometers of the wafer stage ST.

Now, in each of the shot regions SA, the wafer marks for the die-by-dieMD1, MD2, MD3, and MD4 are formed. In the present embodiment, it isassumed that these marks MD1-MD4 are detected by the off-access waferalignment sensors (10-30). Each of the marks MDn should be a multimarkin which four bar marks BPM1, BPM2, BPM3, and BPM4 are arranged inparallel with equal intervals as shown in FIG. 6(a). Also, as shown inFIG. 6(b), the bar marks BPMn are formed convexly on the wafersubstrate. The center Cl of the mark MDn is located between the barmarks BPM2 and BPM3.

Also, FIG. 5 shows the arrangement of the index marks TL and TR on theconjugate index plate 18, and each of the index marks TL and TR isformed with two fine lines of chrome layer on a transparent glass plate.In executing the alignment, the stage ST is positioned by sandwichingthe mark MDn between the two index marks TL and TR. FIG. 7 shows anexample of the video signal waveform thus obtained.

FIG. 7(a) illustrates the state where the wafer mark MDn is sandwichedby the index marks TL and TR, and there is a slight deviation betweenthe center Cl of the wafer mark MDn and the center Ct of the index marksTL and TR. The processor 50 shown in FIG. 3 computes the volume of thisdeviation precisely. As shown in FIG. 7(b), the video signal waveformobtained along the horizontal scanning SL of the CCD 22 becomes bottomminimal value only at the edge positions of each mark because theinterference phenomenon on the resist layer is reduced by the use of thewideband illuminating light. In FIG. 7(b), the index marks TL and TR aretwo fine bar marks respectively. Accordingly, one bar mark thereof hasone bottom waveform BL1, BL2, BR1 and BR2. Also at each of the edgepositions of four bar marks BPM1-BPM4 of the wafer mark MDn, a total ofeight bottom waveforms WL1, WR1, WL2, WR2, WL3, WR3, WL4 and WR4 can beobtained.

Nevertheless, the optical phenomena of the bottom waveforms appearing atthe positions of the index marks TL and TR and the bottom waveformsappearing at the respective edge positions of wafer mark MDn arecompletely different. In other words, the index marks TL and TR arepicked up on the CCD 22 as dark portions because these are illuminatedby transmission by the illuminating light reflected from the wafersurface, whereas each edge of the wafer mark is picked up as darkportion (dark line) because the illuminating light is scattered at anangle larger than the aperture number (N.A.) of the objective lens 12and the like and is not returned in the image-formation optical path tothe CCD 22.

In this respect, the signal waveform shown in FIG. 7(b) is the oneobtained by averaging the signal waveform obtainable along N lines ofscannings SL as shown in FIG. 7(a) after summing such waveform by thepixel columns in the vertical direction. This summing and averaging isexecuted by the processor 50 by reading the waveform data for N linesfrom the RAM 43.

Subsequently, the alignment method according to the present embodimentwill be described, and in the premise thereof, several parameters areset up in the processor 50 in advance. The typical parameters thereofare given below.

1 A center address value ACC for the index marks TL and TR.

2 A distance Lt (μm) between the index marks TL and TR on the wafer.

3 Numbers Kt for each of the index marks TL and TR.

4 Numbers Km for the wafer mark MDn.

5 Point numbers (address) HL and HR from the center address value ACC ofthe index marks TL and TR.

6 Point numbers (address) Pt for each processing width of the indexmarks TL and TR.

7 Point numbers (address) Pm of the processing width from the centeraddress value ACC of the wafer mark MDn.

Of these parameters, the meanings of the point numbers HL, HR, Pt, andPm are illustrated in FIG. 7(a).

Also, for the present embodiment, it is assumed that subsequent to thecompletion of a global alignment of the wafer W, the finer positionaldetection is performed by the use of the wafer alignment sensor.Therefore, if the index marks TL and TR and the wafer mark MDn aredetected by positioning the stage ST only in accordance with thedesigned value of the shot arrangement on the wafer W subsequent to theglobal alignment, a slight irregularity of the shot arrangement or analignment error ΔX including the expansion or contraction of the wafer(W) and the like remains to the extent of the outstanding error (±1 μmor less) at the time of the global alignment. This alignment error ΔX isthe difference between the central positions Cl and Ct shown in FIG. 7.

Now, when the waveform data for N lines of scannings picked up by theCCD 22 are loaded into the RAM 43, the processor 50 executes thewaveform processing in the sequence shown in FIG. 8. Hereinafer, suchprocessing will be described along each of the steps shown in FIG. 8.

[Step 100]

Here, arbitrary lines are selected from the original waveform data of Nlines which have been stored in the RAM 43, and are averaged afteraddition per pixel in the vertical direction to produce one line of theaveraged waveform data. The averaged waveform data thus produced istemporarily stored in the RAM 43.

In this respect, the scannings to be added to obtain its average are notnecessarily continuous in the vertical direction. Such scanning linesmay be obtained at intervals of one line or two lines.

[Step 102]

Subsequently, the processor 50 executes the smoothing of the averagedwaveform data. This smoothing is carried out by allowing the averagedwaveform data to pass through a numerical filter.

FIG. 9A(a) shows an example of the averaged waveform data in the RAM 43,and the horizontal axis represents the address point of the RAM 43 andthe vertical axis represents the level. To this waveform, the numericalfilter FNa shown in FIG. 9A(b) is applied; thus obtained the smoothedwaveform data R(n) by removing the high-frequency component existing inthe averaged waveform data. This waveform data R(n) is also temporarilystored in RAM 43.

[Step 104]

Then, the processor 50 differentiates the averaged waveform data. Thisdifferentiation is carried out by allowing the averaged waveform data topass through the numerical filter FNb having a constant slant as shownin FIG. 9A(c). Thus the bottom waveform shown in FIG. 9A(a) becomes suchdifferentiated waveform data P(n) as shown in FIG. 9B. The address pointPXD which is the bottom point on this differentiated waveform datacoincides with the intermediate point position of the down-slope portionDWS in the averaged waveform data (or smoothed waveform data), and theaddress point PXU which is the peak point on the differentiated waveformdata coincides with the intermediate point position of the up-slopeportion UPS in the averaged waveform data.

Therefore, by executing the differentiation process, all slopingpositions on the smoothed waveform data can be defined. In this respect,the zero crossing point for the differentiated waveform between theaddress points PXD and PXU in FIG. 9B coincides with the bottom point inthe waveform shown in FIG. 9A(a).

[Step 106]

Next, the processor 50 extracts all peak points and bottom points andthe positions thereof in this differentiated waveform data P(n). In thiscase, as shown in FIG. 9B, small bottoms and peaks Dub and Dup otherthan the original bottoms and peaks may also be extracted.

[Step 108]

Then, the processor 50 discards these small bottoms and peaks Dub andDup in the smaller order, and selects the bottom points and peak pointsin the numbers corresponding to the line numbers Kt of the index markand the line numbers Km of the wafer mark.

As shown in FIG. 7 earlier, in the waveform processing width Ptcorresponding to the index marks on the left side and right side TL andTR, it is known that two bottom waveforms are obtainable on the smoothedwaveform data R(n) (index mark line number Kt=2). Therefore, in theprocessing width Pt, two peak points and two bottom points can beobtained on the differentiated waveform data P(n).

On the other hand, in the processing width 2 Pm corresponding to thewafer mark MDn, it is known that eight (2 Km) bottom waveforms areobtainable on the smoothed waveform data R(n). Therefore, in theprocessing width 2 Pm, the eight peak points and eight bottom points canbe obtained on the differentiated waveform data P(n).

With the processings set forth above, the down-slope portions andup-slope portions corresponding to each of the marks on the smoothedwaveform data have been defined.

FIG. 10 represents such state. FIG. 10(a) shows the smoothed waveformdata; FIG. 10(b), the differentiated waveform data; and here, thehorizontal axis of FIG. 10 represents the address points of the smoothedwaveform data to obtain the central positions of the respective slopesin the smoothed waveform data corresponding to the peak points andbottom points on the differentiated waveform data.

The central positions of the respective slopes on the smoothed waveforms(BL1 and BL2) corresponding to the index mark TL on the left side aretwo down-slopes RD(1) and RD(2), and two up-slopes RU(1) and RU(2).Also, the central positions of the respective slopes on the smoothedwaveform (BR1 and BR2) corresponding to the index mark TR on the rightside are two down-slopes RD(3) and RD(4) and two up-slopes RU(3) andRU(4).

Likewise, the central positions of the respective slopes on the smoothedwaveforms generated at the respective edges of the four bar marks BPM1BPM2 are down-slopes WD(1)-WD(8) and up-slopes WU(1)-WU(8).

Here, it is desirable as a method to define the down-slope and up-slopethat the contrast limit is established in practice by the use of therespective contrast values (levels) between the smoothed waveforms anddifferentiated waveforms, and that the respective slope positions aredefined in the smoothed waveforms based on the such limit value.

FIG. 11(a) is the enlarged representation of the bottom waveform WL1only in those shown in FIG. 10(a). FIG. 11(b) is the enlargedrepresentation of only the differentiated waveforms in FIG. 11(a).

At first, the absolute value of the differentiated level (contrastvalue) CWD(1) corresponding to the bottom position WD(1) in thedifferentiated waveform data is obtained. Then, the level CDS(1) in thesmoothed waveform corresponding to the position WD(1) is obtained. Thislevel CDS(1) is registered as a value slightly smaller than the level inthe down-slope defined by the position CWD(1).

Subsequently, the processor works out the contrast value CVW(1) by anequation given below.

    CVWd(1)=A.CDS(1)+B.CWD(1)

Likewise, the absolute value of the differentiated level CWU(1)corresponding to the peak position WU(1) in the differentiated waveformdata is obtained. Then, the level CUS(1) in the smoothed waveformcorresponding to the position WU(1) is further obtained.

Subsequently, by an equation given below the contrast value CVWu(1) isobtained.

    CVWu(1)=A.CUS(1)+B.CWU(1)

Here A and B are constant. However, if noise should be discriminated,these are set as A=1, B=approximately 0.5.

The above-mentioned operations are executed in the signal processingregion for the wafer mark and at the same time, exactly the sameoperations are executed for the signal waveform of the index mark.

As to the index mark, its bottom position in the differentiated waveformis RD(1), and peak position, RU(1) if the bottom waveform BL1 in FIG.10(a) is taken as an example.

Then, assuming that the level (bottom) in the differentiated waveform atthe position RD(1) is CFD(1); the level (peak) in the differentiatedwaveform at the position RU(1) is CFU(1); and the level in the vicinityof the center of the down-slope of the bottom waveform BL1 in thesmoothed waveform is CDR(1); and the level in the vicinity of theup-slope is CUR(l), the contrast values CVRd(1) and CVRu(1) of the indexmark are obtained respectively as follows:

    CVRd(1)=A.CDR(1)+B.CFD(1)

    CVRu(1)=A.CUR(1)+B.CFU(1)

Then, the processor obtains the contrast ratio GG of the the wafer markto the index mark by an equation given below.

    GG=CVWd(1)/CVRd(1)×100 (%)

or

    GG=CVWu(1)/CVRu(1)×100 (%)

Then, if this contrast ratio GG is less than the predetermined ratio,the processor judges that the bottom waveform is not the one whichcorresponds to the edge of the wafer mark.

[Step 110]

Subsequently, the processor 50 compares the respective slope portions inthe smoothed waveform with the predetermined slice level to obtain itsintersections. This step 110 may be omitted in some cases becausesometimes it is possible to use the central positions of the respectiveslopes on the smoothed waveform obtained as shown in FIG. 10 as they arein the processes to follow thereafter.

Now, at this step 110, the optimum slice level is determined for each ofthe slopes as described earlier in conjunction with FIG. 2(c). When thisslice level is determined, each of the up-slope positions RU(1)-RU(4)and down-slope positions RD(1)-RD(4) of the index mark obtained earlierin FIG. 10 and up-slope positions WU(1)-WU(8) and down-slope positionsWD(1)-WD(8) of the wafer mark are used. Now, an specific example will bedescribed in reference to FIGS. 12A and 12B. First, as shown in FIG.12A, the waveform data are searched fore and after in predeterminedportion of point numbers (address) from the down slope position WD(1) ofone bottom waveform WL1 on the smoothed waveform. Then, the minimumvalue BT of the bottom of the down-slope and the maximum value SPd ofthe shoulder of the down-slope are obtained, and as shown in FIG. 12B,the slice level S1 is defined at a location where the value between theminimum value BT and the maximum value SPd is divided by thepredetermined ratio.

Here, given the ratio thereof as α(%), the slice level S1 is worked outby an equation given below.

    S1=(SPd-BT)×(α/100)+BT

Subsequently, the level position of the down-slope portion whichcoincides with this slice level S1 is obtained. At this juncture, if thelevel which coincides with the slice level S1 exists between thesampling points, the intersecting point SWD(1) is obtained by a methodof the linear interpolation or the like. This position SWD(1) is, forexample, represented by a real number which is obtained by interpolatingthe space between the address points with a 1/10.

In the same way as above, a search is carried out fore and after fromthe position WU(1) for the up-slope of the bottom waveform WL1 on thesmoothed waveform (here, since the minimum value BT has been known, thesearch may be performed for one direction only), and the slice level S2is defined by an equation given below.

    S2=(SPu-BT)×(α/100)+BT

Then, the position SWU(1) of the up-slope portion which coincides withthis slice level S2 is worked out by a real number.

Thereafter, the optimum slice levels for each of the bottom waveforms inthe smoothed waveform are defined in the same fashion to obtain theintersecting points SRU(1)-SRU(4), SRD(1)-SRD(4), SWU(1)-SWU(8), andSWD(1)-SWD(8).

[Step 112]

Next, the processor 50 examines one pixel of the CCD 22 (the spacebetween the samplings of the smoothed waveform data) to work out itslength in terms of μm on the surface of the wafer in order to cancel anymagnification error and the like of the optical system of the waferalignment sensor, and obtains the converted value UNT thereof (μm/point)in a real number. Here, it is assumed that a space Lt (μm) for designingthe index marks TL and TR having excellent stability is applied. Thespace Lt is registered as a value on the wafer surface and should beconverted to its UNT value by an equation given below. In this respect,both index marks TL and TR are assumed to be Kt lines (in the presentinvention, Kt=2). ##EQU1##

[Step 114]

Subsequently, the processor 50 obtains the central position Ct(μm)between the index marks TL and TR in a real number in accordance with anequation given below. ##EQU2##

[Step 116]

Here, an algorithm for working out the central position Cl of the wafermark is selected in accordance the processing mode designated inadvance. The step 116 to proceed to the next process (either one of thesteps 118, 120, and 122) is instructed by an operator or isautomatically switched by the auto-set-up system.

[Step 118]

Here, the central position Cl(μm) of the wafer mark is worked out in areal number by the inner slope detection method.

Now, the inner slope positions on the wafer mark waveform are SWU(1),SWD(2), SWU(3), SWD(4), SWU(5), SWD(6), SWU(7), and SWD(8) in referenceto FIG. 10 described earlier.

Therefore, give the numbers of the wafer mark as Km here (in the presentembodiment, Km=4), the central position Cl is worked out in accordancewith an equation given below. ##EQU3##

[Step 120]

Here, the central position Cl(μm) of the wafer mark is worked out in areal number by the outer slope detection method.

Now, the outer slope positions of the wafer mark are SWD(1), SWU(2),SWD(3), SWU(4), SWD(5), SWU(6), SWD(7), and SWU(8) in reference to FIG.10 described earlier.

Therefore, the central position Cl is worked out herein accordance withan equation given below. ##EQU4##

[Step 122]

Here, the central position Cl(μm) of the wafer mark is worked out in areal number by both of the slope detection methods.

As clear from FIG. 10 described earlier, the average position obtainedafter adding all the down-slopes and up-slopes on the wafer markwaveform becomes the center Cl. Therefore the center Cl is worked out inaccordance with an equation given below. ##EQU5##

[Step 124]

Subsequently, the processor 50 determines the alignment error ΔA(μm) bycomputing the difference between the central position Ct of the indexmark and the central position Cl of the wafer mark.

This alignment error ΔA is the remaining alignment error of the waferstage ST when the video signal waveform has been stored in the RAM 43.Therefore, in positioning the stage ST thereafter, it suffices if onlythe designed value of the stage positioning coordinate defined by theglobal alignment is offset by ΔA.

So far the fundamental alignment procedures for the present embodimenthave been described. Now, an example will be described as to theselection of the processing mode at the step 116 according to thepresent embodiment.

Usually, among the processes to form a device on the semiconductorwafer, there is a process to deposit aluminum layer evenly thereon toproduce wirings and others between the elements, and the irregularity ofthe alignment mark on the wafer is detected in such a state where themark is covered by the aluminum layer. In other words, the detection isperformed against the mark which is the aluminum layer itself.

As a result, if the aluminum layer is not deposited on the mark evenlyand the mark becomes asymmetrical, the video signal waveform (bottomwaveform) corresponding to the edge portion at each end of the mark alsobecomes asymmetrical. FIG. 13(a) shows the sectional structure of thealignment mark WM covered by the aluminum layer Al, and on the markimage projected on the screen of the television monitor, which has beenpicked up by the CCD 22, the width of the dark lines appearing on theedge portions on the left and right sides differs from each other asshown in FIG. 13(b).

This is due to the fact that the deposition of the aluminum layer Al isasymmetrical at the left and right edge portions of the mark WM as shownin FIG. 13(a). If this mark WM is observed by the use of a visible bandilluminating light, it is usual that only the surface of the aluminumlayer Al is seen. Accordingly, the video signal waveform output by theCCD 22 becomes as shown in FIG. 13(c), and the bottom waveformscorresponding to the left and right edge portions also become differentfrom each other.

To a waveform such as this, the signal waveform processing algorithm ofthe present embodiment is applied to obtain the outer slope positionsSWD(1), SWU(2), and the inner slope positions SWU(1), SWD(2). Then, if,at the step 122 shown in FIG. 8, both of the slope detection methods areselected, the central position Cl of the mark WM shown in FIG. 13 isobtained by an equation given below.

    Cl={SWD(1)+SWU(2)+SWU(1)+SWD(2)}/4

Nevertheless, it has been ascertained that when a mark with aconspicuous asymmetry such as this is detected by both of the slopedetection methods to execute its alignment, the resultant precision isnot necessarily sufficient.

One reason for this is that there is a problem for a vernier whichexamines the alignment accuracy. (overlay accuracy).

In examining the overlay accuracy, the vernier on the reticle ispositioned against the main scale of the vernier provided in advance onthe wafer by the use of the alignment sensor to produce an over print ofthe vernier. Then, the alignment accuracy is judged by reading thedegree of the deviation of the verniers produced by the over print.

Conventionally, this examination is carried out in such a manner thatsubsequent to the development of the wafer over printed by the use ofstepper, the vernier formed by the resist and the main scale of thevernier on the substrate are observed by another optical microscope orthe like to read the degree of the deviation of the verniers byeye-sight.

FIG. 14 and FIG. 15 illustrate an example of the vernier on the aluminumlayer. FIG. 14 shows the case where the vernier WBS is penetratinglyformed in the resist layer PR over the main scale of the vernier WBM.FIG. 15 shows the case where two verniers WBS are penetratingly formedin the resist layer PR over both sides of the main scale of the vernierWBM.

Here, the main scale of the vernier WBM is assumed to be asymmetric.

When these verniers are measured by eye-sight, the distances a and bbetween the edge portions of the vernier WBS in the resist and theadjacent edge portions of the main scale of the vernier respectively areread, and the position where these distances are identical by eye-sightis regarded as indicating the alignment accuracy.

Specifically, as shown in FIG. 16, the main scale WBM is produced withconstant pitches in the measuring direction, and the vernier WBS to beoverprinted thereon is provided with pitches larger than those of themain scale WBM by 0.02 μm each, for example. If the alignment is ideallyperformed, the centers of the main scale WBM and the vernier WBS areoverlapped with each other at the position represented by a numeral 0accompanying the vernier. In the case shown in FIG. 16, the centers ofthe main scale WBM and the vernier WBS are overlapped at the positionrepresented by a numeral -0.2. Hence, the alignment accuracy obtained is-0.02 μm. In this respect, FIG. 16 illustrates the vernier patterns ofthe method shown in FIG. 14. The same is applicable to the method shownin FIG. 15.

Now, in the case of the vernier type shown in FIG. 14, the edgepositions on the main scale WBM which regulate the distances a and b arethe outer slope positions SWU(1) and SWD(2) if these edges should belocated on the waveform shown in FIG. 13(c).

On the other hand, the vernier type shown in FIG. 15, the edge positionson the main scale WBM which regulate the distances a and b are the outerslope positions SWD(1) and SWU(2) if located on the waveform shown inFIG. 13(c).

In other words, when the actual alignment is performed it is necessaryto select the outer slope detection method or the inner slope detectionmethod by the vernier type that has been used for checking the alignmentaccuracy.

Therefore, in the case of the alignment checking by the vernier typeshown in FIG. 14 (FIG. 16), the inner slope detection method (the step118 in FIG. 8) is selected, and in the case of the vernier type shown inFIG. 15, the outer slope detection (the step 120 in FIG. 8) should beselected.

In this way, it is possible to coordinate precisely the alignmentaccuracy measured by the vernier scale with eye-sight and the alignmenterror detected by the wafer alignment sensor.

However, depending on the ways of process, the alignment shouldsometimes be performed with the mark WM under the aluminum layer Al. Insuch a case, it is difficult to define the degree of asymmetry of thealuminum layer Al formed on the mark WM. Therefore, subsequent to theverification of the asymmetry thereof by examining the sectionalstructure of the mark, an automatic selection should be arranged toprovide a more weight on the inner slope detection method or on theouter slope detection method in accordance with the degree of theasymmetry thus verified. For example, the central position of the markCl for a single mark waveform such as shown in FIG. 13(c) is determinedby an equation given below. ##EQU6##

This equation is a modification of the equation for the detection methodfor both slopes with the insertion of the weighted constants A and B,and if only the condition given below is satisfied, the constants A andB are applicable.

    0<A<2, 0<B<2, A+B=2

Here, if the weighted constants A and B are given as 1 for both of them,then the related equation is for the detection method for both slopes.

In this respect, as a method of examining the sectional structure of themark, there are considered such method as using a scanning electronmicroscope (SEM) measuring machine or a ultrasonic microscope, or suchmethod as applying an optical measurement by the use of an infraredlaser spot or illuminating light capable of transmitting itself throughthe aluminum layer Al.

Now, the asymmetry of the aluminum layer Al, when it is deposited, tendsto expand isotropically from the center of the wafer, and it is possibleto recognize the asymmetry as shown in FIG. 17, for example, at theposition where the shot (chip) marks positioned at the periphery of thewafer surface are observed by eye-sight through the wafer alignmentsensor.

FIG. 17 shows the four shot positions at the periphery on the shotarrangement coordinate XY with the wafer center as its substantial homeposition. For each shot, the marks respectively for the X directionalignment and Y direction alignment are provided. For the two shotspositioned apart from each other in the Y axis direction on thecoordinate XY, the mark MDy for the Y direction alignment is observedwhile, for the two shots apart from each other in the X axis direction,the mark MDx for the X direction alignment is observed.

At this juncture, the signal waveform of each mark picked up by the CCD22 is processed to obtain the width of the bottom waveform at the markedge portions, i.e., the difference between the positions SWD(1) andSWU(1) shown in FIG. 13(c), and the difference between the positionsSWD(2) and SWU(2). Hence, it becomes clear that there is an intensifiedasymmetry on the edge having the larger difference. The amount of thisasymmetry ΔU is obtainable by an equation given below. ##EQU7##

In this way, some of the shot marks on the periphery of the wafer aredetected, and obtaining the amounts of the asymmetry ΔU at thosepositions makes it possible to define the asymmetry at the time of thealuminum layer deposition almost over the entire surface of the wafer.

Therefore, as shown in FIG. 1, in the stepper provided with thedetection system to detect the die-by-die mark on the reticle R and themark for one shot portion on the wafer W with the TTR alignment systemDAS1-DAS4, it becomes possible to correct the wafer mark positionaligned by the TTR alignment system in response to the asymmetry of themark.

Here, as an example of the TTR alignment system, the interferencealignment system disclosed in U.S. Pat. No. 5,004,348 is considered.

FIG. 18 is a view schematically showing an interference alignment systemwhich is, through slightly different from the one disclosed in this U.S.patent, identical in its principle.

On the reticle R, refraction gratings Gr1 and Gr2 are provided apartfrom each other in the grating pitch direction in two transparentwindows as die-by-die marks, and two laser beams Lf1 and Lf2 having thewavelength different from the exposure light are slantly irradiated ontothe gratings Gr1 and Gr2 respectively. The major rays of light for thebeams Lf1 and Lf2 are intersected in the space above the reticle R, andthe space between the optical axis directions of such intersecting pointand reticle R corresponds to the the amount of the chromatic aberrationon the axis of the projection lens in the wavelength of the beams Lf1and Lf2. The beams Lf1 and Lf2 transmitting themselves through thetransversal transparent portions of the gratings Gr1 and Gr2 on thereticle R intersect with each other on the wafer W through theprojection lens. In the intersecting region, one-dimensionalinterference fringe is produced in parallel with the refraction gratingGw on the wafer W. From the grating Gw on the wafer W, the coherentlight BTL interfered by ±primary refraction light is generatedvertically, and this coherent light BTL reversely advance in theprojection lens to be photoelectrically converted through the center ofthe transparent window of the reticle R. Here, if a frequencydifferential Δf is given to the two beams Lf1 and Lf2, the interferencestrip formed on the grating Gw of the wafer W flows at a speed inresponse to the frequency differential Δf, and the photoelectricalsignal (measuring signal) of the coherent light BTL becomes thealternating current signal which changes into a sine wave skate by thefrequency Δf.

Meanwhile, from the grating Gr1 and Gr2 of the reticle R, the ±primaryrefraction lights DL1 and DL2 are generated in the reverse direction tothe light transmitting beams Lf1 and Lf2, and the reference signal isproduced by photoelectrically detecting the coherent light interferingthese ±primary refraction lights DL1 and DL2.

This reference signal also becomes the alternating current signal whichchanges into the sine wave state by the frequency Δf, and the phasedifference Δφ (within ±180°) between the reference signal and themeasuring signal becomes the amount of the deviation in the pitchdirection between the gratings Gr1 and Gr2 of the reticle R and thegrating Gw of the wafer W. The system in which the frequencydifferential Δf is given to the two beams Lf1 and Lf2 in this way iscalled especially the heterodyne interference alignment system which iscapable of detecting a positional deviation of approximately ±0.01 μm asa system thereby obtaining a phase differential measuring resolution of±2° because the maximum phase differential ±180° corresponds to ±1 μmwhen the pitch of the grating Gw is set to be approximately 4 μm(line-and-space of 2 μm wide).

Now, if a high-precision, high-resolution TTR alignment sensor is usedand there is an asymmetry generated for each of the grating elements ofthe grating mark Gw on the wafer W, error (offset) is included in theresultant mark position detection as a matter of course. Subsequently,therefore, a method of offset correction will be described, in which theasymmetry of the mark presenting a problem in a TTR alignment system ofthe kind is estimated for the correction by the wafer alignment sensorusing a wideband illuminating light.

FIG. 19(a) shows the sectional shape of the grating mark Gw on the waferW, and the edge on the right side of each of the grating elements isdeformed. As a result, even if the TTR alignment system shown in FIG. 18is used to perform the alignment with the gratings Gr1 and Gr2 of thereticle R by flowing the interference strip IF to detect the gratingmark Gw with the helerodyne detection, there still remains an offsetsuch as averaging the sum of the asymmetry of each of the gratingelements.

Therefore, in the same way as the embodiment described earlier, thegrating mark Gw is picked up by the CCD 22. At this juncture, thehorizontal scanning direction of the CCD is arranged to be in parallelwith the pitch direction of the grating mark Gw. Thus, as shown in FIG.19(b), the video signal waveform from the CCD 22 becomes the bottomwaveform asymmetrical at the edge portions of both sides of each gratingelement. Then, as described in conjunction with FIG. 13, the down-slopeposition SWD(n) and up-slope position SWU(n) are obtained. Further, whenthe amount of asymmetry ΔU (n) is worked out for each of the gratingelements and is averaged, the amount of the asymmetry as a whole isobtained for the grating mark Gw. Therefore, at the time of thedie-by-die alignment, if the alignment is performed by the TTR alignmentsystem in accordance with its resultant mark position detection and theoffset based on this amount thus worked out, it is possible to reduceerror due to the asymmetry of the mark even when a TTR alignment systemusing a single wavelength illuminating beam is employed.

Next, in reference to FIGS. 20A, 20B and 20C, the description will bemade of the case where the clear bottom waveform does not appear at theedge portions of the alignment mark in terms of the signal processalgorithm.

FIG. 20A shows the case where the reflection factor of the multimark MD(convex portion) on the wafer is extremely different from its peripheralreflection factor. The signal waveform at this time shows a shape of thewaveform corresponding to the contrast difference between the mark andthe substrate.

FIG. 20B shows the case where the line and space duty of the multimarkMD is set to be a value other than 50%, and if the line width of theadjacent covex bar mark is narrow, the bottom waveform at the left andright edges is not separated and becomes a single bottom waveform.

Also, FIG. 20C shows the case where each of the bar marks of themultimark MD is formed by a square dot to constract the grating. In thiscase, it is also impossible to obtain a clear bottom waveform. Thewaveform becomes a short form wave.

In either case of those shown in FIGS. 20A, 20B, and 20C, the innerslope detection method cannot be utilized. Only the outer slopedetection method is employed. As described in the embodiment earlier,the wafer mark lines Km is defined in advance as an operation for thealgorithm, and it is assumed that the bottom waveform having a constantcontrast on the signal waveform is obtained only for the number 2 Km. Asa result, an error tends to occur in the algorithm (operation) if thebottom waveform generated at the mark edge portions is not clear.

Therefore, a routine for judging the contrast is added to the flowchartshown in FIG. 8, so that the processing system automatically selects thestep 120 in FIG. 8 if the signal waveform shows the state as shown inFIG. 20.

FIG. 21 is a flowchart showing an example of such contrast judgementroutine, which is executed in place of the step 116 shown in FIG. 8.

Hereinafter each of the steps shown in FIG. 21 will be described.

[Step 200]

Here, zero is set for the inner counter (software counter) FN of theprocessor. This counter FN is provided for discriminating the waveformshown in FIG. 20 from the normal waveform shown in FIG. 10.

[Step 202]

Here, assuming that the waveform shown in FIG. 22 is obtained, thedescription will be made.

At first, since the down-slope position SWD(n) or WD(n) in the waveformshown in FIG. 22 has been obtained, the contrast values (levels) CVl andCVr each at a predetermined position to the left and right therefrom areobtained. The predetermined distance of this position is substantiallyequal to or slightly longer than the width of the normal bottom waveformat the edge.

[Step 204]

Subsequently, the processor works out the difference between thecontrast values CVl and CVr, and judges whether the difference is largerthan a predetermined value GC or not.

The first bottom waveform in FIG. 22 is normal, corresponding to themark edge portion only. Accordingly, the difference between the contrastvalues CVl and CVr is not so large, and the process proceeds to the step206.

[Step 206]

Here, the content of the counter FN is incremented by 1 (+1).

[Step 208]

The processor judges whether all down-slope positions SWD(n) have beenchecked or not, and if the check has not been completed, the processorinstructs a jump to the step 202 to execute the same process for thenext down-slope.

[Step 210]

Here the processor judges whether or not the content of the counter FNstill remains zero. The counter FN is not incremented in a state such asthe down-slope position SWD(2) being in FIG. 22, i.e., the differencebetween the contrast value CVl and CVr at fore and after of the positionSWD(2) is greater than the value GC. Consequently, if the counter FN iszero, it is meant that the signal waveform is in the condition shown inFIG. 20, and the processor executes automatically (forcibly) the step120 to perform the outer slope detection.

[Step 212]

Also, if the counter FN is not zero, the processor compares the countedvalue with the wafer mark line number Km, and judges that the signalwaveform is in the condition shown in FIG. 22 if the value and thenumber do not coincide. Then, the step 118 is executed to perform theinner slope detection.

Further, if the value of the counter FN coincides with the mark numberKm, the processor judges that the normal bottom waveform has beengenerated with respect to all the mark edge portions, and executes aprocessing mode (either one of the three slope detection methods)designated in advance by the user (operator). It is apparent that if thevalue of the counter FN does not coincide with the mark number Km, aprocessing mode employing outer slope detection (as well as a processingmode employing both outer and inner slope detection) is prohibited.

As the above describes, even when such waveform as shown in FIG. 20 isobtained, the process can be executed without errors in terms of thealgorithm. Nevertheless, in the case of the mark shown in FIG. 20, onlythe outer slope detection method is usuable. Therefore, even if theinner slope detection method is found to be optimum in consideration ofthe asymmetry based on the Vernier configuration as described inconjunction with FIG. 14 and FIG. 15, this is not usable after all. Forexample, in the case of a multimark having one extremely narrow convexbar mark as shown in FIG. 20C or FIG. 20B, there appears conspicuous thedifference affected by the asymmetry based on the vernier configuration.

In such a case, therefore, it is possible to utilize an optimum slopedetection method to be determined by the vernier configuration byreplacing the convex bar mark with a concave bar mark.

In this respect, if the multimark of line-and space as shown in FIG. 20Bearlier is used, only one bottom waveform is generated for one bar mark.The arrangement can be made so as to obtain the bottom waveform which isseparated at the edges of both sides of one bar mark while changing theduty ratio of the line-and-space. This method is effectively applicableto the wafer grating Gw of the interference alignment system shown inFIG. 19. In the interference alignment system, the narrower the pitch ofthe grating mark Gw, the higher becomes its resolution. However, for thewafer alignment sensor using the CCD 22, if the pitch of the gratingmark Gw is narrower, the waveform of the video signal becomes such asshown in FIG. 20A, and the contrast is further worsened. Therefore, bychanging the duty ratio without changing the pitch of the grating markGw, it is possible to produce the waveform of the video signal as closeas possible to the one shown in FIG. 19 or FIG. 20B.

In the apparatus of the present embodiment, the illuminating light forobserving the wafer mark is wideband, and there is no interferencephenomenon due to the resist layer at all. Therefore, in order toincrease the resolution (magnifying power), it is possible to make lowerthe aperture number (N.A.) of the optical system (objective lens 12)before the CCD 22. Then, it becomes impossible to obtain any practicabledepth of focus. Therefore, the aperture number of the objective lens 12is set to be approximately half of the projection lens PL,N.A.=approximately 0.2-0.3, for example. Further, the totalimage-formation magnifying power, which is determined by the opticalsystem (12 and 16) from the wafer surface to the conjugate index plate18 and the optical system (20) from the conjugate index plate 18 to theCCD 22, is set to be approximately 30-50. In this way, even when theline-and-space of the practicable multimark is set to be 4 μm (pitch 8μm), there is no split top phenomenon appearing in the bottom waveformon the video signal waveform with respect to the mark edge portion. Thesplit top phenomenon means that in consideration of the cross-section ofthe convex bar mark shown in FIG. 23(a), each of the bottom edges (outeredges) BE1 and BE2 and the top edges (inner edges) TE1 and TE2 isseparated to be the bottom waveforms BWB1, BWB2, BWT1, and BWT2 as shownin FIG. 23(b). This is caused by the fact that even when theilluminating light IL is irradiated in the direction perpendicular tothe edge taper portion between the bottom edge BE1 (BE2) and the topedge TE1 (TE2), the scattered rays of light DFL from the tapered portionare returned to the CCD 22 if the aperture number of the objective lens12 is large and its magnifying power is high.

Consequently, if the video signal shown in FIG. 23(b), which is suppliedto a television monitor, is observed on its screen, the edge portions ofthe bar mark look like two fine black lines.

When the signal waveform with the split top phenomenon is processed,there is some case where the separated bottom waveforms BWB1 and BWT1are erroneously recognized as two edges,

In the apparatus of the present embodiment, the configuration changes ofthe wafer mark in the course of processes are experimentally consideredso as not to create such split top phenomenon as this, and the aperturenumber of the objective lens 12 is set comparatively small to be 0.2-0.3and the magnifing power to the CCD 22 is set to be 30-50. Further, thecell size (cell pitch) of the CCD 22 is approximately 0.2 μm-0.3 μm interms of the wafer surface.

Subsequently, in reference to FIG. 24 and FIG. 25, the systemconfiguration of a second embodiment according to the present inventionwill be described. In the present embodiment, the conjugate index plate18, the structure of the CCD 22, and the method of the wafer markalignment differ from those in the previous embodiment. FIG. 24 showsthe system in the case where the mark in the X direction on the wafer Wand the one in the Y direction thereon are detected through a commonoptical system, and what differs from FIG. 1 is that two sets of indexmark groups are formed on the index plate 18, each in the X directionand Y direction thereon; a beam splitter 21 is provided after theimage-formation lens system 20 to divide the image-formation beam intotwo; and two CCD's 22X and 22Y are provided to receive such dividedimage-formation beams respectively. However, the two CCD's 22X and 22Yare set up so that the horizontal scanning directions thereof are atangles of 90° to each other as indicated by arrow.

Further the conjugate index plate 18 is provided, as shown in FIG. 25,with a region VPBx including the index mark groups TLA, TRA, TLB, andTRB in the X direction, and a transparent region VPAx above them, and amark VCMx for eye-sight. Likewise, in the Y direction, the index markgroups TLA, TRA, TLB, and TRB, and a mark CVM_(y) for eye-sight areprovided.

The CCD 22X covers the regions VPAx and VPBx, and the mark VCMx, and isprovided with a pick-up area which does not take in the index marks TRAand TLA in the Y direction. The same is applicable to the CCD 22Y. Inthe present embodiment, the conjugate index plate 18, and the system upto the image-formation lens system 20 are used in common both in the Xand Y directions. Accordingly, the mirror 10 to observe the wafersurface, and the objective lens 12 are arranged at one location only.

In this respect, if the alignment optical system for the X direction andY direction is each arranged independent of the objective lens, theconjugate index plate 18 is also separated for the X direction use and Ydirection use as a matter of course.

Now, the inner index marks TLA and TRA among those conjugate index markgroups shown in FIG. 25 are produced as an example to sandwich amultimark having seven bar marks of each 4 μm wide at intervals of 4 μmspace. Therefore, in a case of detecting a single mark, not multimark,or the like, the wafer surface below each of the index marks TRA and TLAinevitably becomes a prohibitive region for marks or patterns. In otherwords, the formation region for the wafer mark should be provide widelyon the street line, which restricts device fabrications.

In the present embodiment, therefore, at the time of detecting a singlemark for the X direction use, the arrangement is made to sandwich thesingle mark between the index marks TRA and TRB on the right side ofFIG. 25, and the video waveform portion including the index marks TRAand TRB only is processed.

Also, for a wide mark, the index marks TLB and TRB may be used.

Specifically, as shown in FIG. 26, the single mark WD is sandwiched bythe index marks TRA and TRB, and from the averaged waveform obtained byadding the video signals of n line scannings, the waveform portions inthe index mark processing regions R-L and R-R given as parameter inadvance and the waveform portion in the wafer mark processing region W-Abetween them are selected to form the signal waveform in the same way asin the previous first embodiment. Also, as to the multimark which iswider as a whole, the index mark processing regions R-L and R-R shouldbe set up so as to use the outer index marks TLB and TRB as shown inFIG. 27, and the wafer mark processing region W-A should be set up so asto remove the wafer mark waveform portion which is overlapped with theinner index marks TLA and TRA. The set up of these processing regions isautomatically executed by registering the mark configurations anddimensions to be used in advance.

Also, there is some case where the mark is overlapped with the indexmark to be used depending on the contour of the registered mark. It maybe possible to avoid such overlapping with the index mark byintentionally shifting the specific wafer mark position in the X or Ydirection (measuring direction) subsequent to the wafer globalalignment.

Next, a third embodiment will be described. Here, the description ismade of the case where the wafer alignment sensor of the off-axis typeshown in FIG. 1 is utilized for the wafer global alignment.

In general, a stepper of the kind is used to detect the orientation flatof a wafer to position the wafer mechanically (prealignment), and mountit on the stage ST. In such state, however, a prealignment error ofapproximately 20 μm-100 μm exists. The global alignment is a process tosearch the global alignment marks on the wafer with the prealignmenterror in view and coordinate the actual shot arrangement with thedesigned shot arrangement within the error range of approximately ±1 μm.Therefore, when a CCD camera is used for the global alignment, there maybe some case where the global mark is not found in the pick-up region ofthe CCD camera if the prealignment error is great even when the stage STis positioned by the designed value.

Then, it is necessary to perform the global search thereby to observethe wafer surface with CCD and shift the wafer gradually by apredetermined amount when the global alignment is carried out for thewafer W by picking up the wafer surface by the CCD camera. For thispurpose, the transparent region VPAx (or VPAy) of the index plate 18shown in FIG. 25 is used. Since this region VPAx is positioned inadvance at a predetermined location on the pick-up surface for the CCD22X, the scanning positions and lines for scanning the region VPAx areknown in advance. Also, the global mark WGM on the wafer should beformed within the street line SAL shown in FIG. 28.

This global mark WGM is formed with three grating type marks arranged inparallel along the Y direction in which the street line SAL extends, andthe distance from the chip region CPA on the left side of the streetline SAL to the first grating type mark is designated by a referencemark a, while the distance from the chip region CPA on the right side tothe third grating type mark is designated by a reference mark d.Further, the spaces between the three grating type marks are b and crespectively.

Here, it is assumed that as shown in FIG. 28, the transparent regionVPAx of the index plate 18 is placed mainly over the chip region CPA onthe left side when the wafer stage ST is initially positioned inaccordance with the designed value to take in the first and secondcolumns of the global mark WGM. At this juncture, if the video signalscorresponding to plural lines of scannings in the region VPAx are addedto obtain its average, such waveform data as shown in FIG. 29(a) arestored in the memory.

Subsequently, the waveform data initially stored are analyzed to verifywhether these area for the global mark WGA or not. As an algorithm forsuch verification, the method disclosed in U.S. Pat. No. 4,723,221 isapplicable, for example.

In other words, the waveform position which is the closest to the stateof the positioned relationship (spaces a, b, c, and d) of the designedmark WGM shown in FIG. 28 is searched.

Usually, three columns of the mark WGM are included in the waveform datainitially stored as in FIG. 29(a). However, if the prealignment error isextremely great, the region VPAx does not cover the mark WGM in thethird column as shown in FIG. 28.

Thus, the processor stores in the memory the video signal waveform fromthe CCD camera subsequent to shifting the wafer stage ST in the Xdirection for a predetermined amount. At this juncture, the region VPAxis shifted to the right side of FIG. 28 to overlap itself partially withthe initial portion. The averaged waveform obtained by the video signalsfrom the region VPAx which has been shifted to the right side is such asshown in FIG. 29(b). In FIG. 29, the overlapped region in the Xdirection in the region VPAx is DBA, and although this length can beaccurately set up by the interferometer IFX of the stage ST, it isdesirable to define the region DBA to be slightly larger the width ofthe mark WGA in the X direction (approximately b+c).

Next, the processor compares the contrast value CVa of the overlappingregion DBA of the video signal waveform which has been stored for thefirst time with the contrast value CVb of the overlapping region DBA ofthe video signal waveform which is stored for the second time.

In general, the CCD camera causes its AGC (auto-gain control) to operatewhen the average luminance of the image plane changes. Therefore, thecontrast values CVa and CVb of the two waveform portions in theoverlapping region DBA may change.

Therefore, if the two contrast values CVa and CVb differ greatly, thegain of either one of the first and second video signal waveforms iscompensated by computation. Then, the two video signal waveforms arejoined in the overlapping region DBA after averaging them. This processis executed by the processor operating the data stored in the memory.

Thus, if the video signal waveforms are joined by shifting the regionVPAx relatively in the X direction, it is possible to store in thememory the video signal waveforms in succession from the region which isfar wider than one single image plane of the CCD camera. Accordingly, itis possible to search the global mark WGM in the street line SAL basedon the design rule (spaces a, b, c, and d).

As the above describes, the search on the global mark WGM is completedwhen the three columns of the mark have been recognized. Then,subsequent to this search, the processing proceeds to the global finealignment. There are several examples of variations of the global finealignment. Roughly, there are systems utilizing the wafer alignmentsensor with the CCD camera employed for the present embodiment as it is,and the alignment sensor which is separately provided for the finealignment.

In the case where the wafer alignment sensor with the CCD camera isutilized, the wafer stage ST is moved to arrange the global mark WGM inthe region VPBx (FIG. 25) of the index plate 18 for the storage of thevideo signal waveform. Then, the alignment is precisely carried out bysandwiching index marks TLA and TRA or sandwiching the second column(single mark) of mark WGM with the index marks TRA and TRB.

Also, in the case of using the fine global sensor which is separatelyprovided, only the second column of the mark WGM is immediatelydetected, and the coordinating value of the stage ST at which thedetected center of the sensor and the center of the second column of themark coincide with each other should be measured.

Subsequently, a forth embodiment will be described. Here, thedescription will be made of the case where the wafer alignment sensor ofthe off-axis type shown in FIG. 1 is utilized for E.G.A.(enhanced-global-alignment).

As regards the E.G.A., there is a detailed disclosure in U.S. Pat. No.4,780,617. Here, therefore, the description of the detailed operationmethod thereof is omitted.

FIG. 30 shows only the shots S1-S8 which are given the sample alignmentof E.G.A. system among the shot arrangement on the wafer. Traditionally,in the E.G.A. system, the sample alignment of the shots S1-S8 isexecuted subsequent to the completion of the global alignments in the X,Y, and θ directions, which is the prerequisite of this system.

In the present embodiment, the global alignment function in the 8direction is included in the E.G.A. sequence to improve its throughput.In the usual E.G.A., the marks in the X direction and Y direction foreach shot are detected one after another in the sequence of the shotsS1-S8 to measure the central coordinating value of each shot. In thepresent embodiment, however, the sample alignment is performed for thosehaving substantially the point symmetry on the wafer in the first twoshots. Specifically, those two are the shots S3 and S7 aligned in the Xdirection or shots S1 and S5 aligned in the Y direction in FIG. 30.

Then, when the sample alignment has been completed for the two shots,the rotation volume Δθ for the XY coordinate system for the wafer (shotarrangement) as a whole in worked out. Then, if this rotation volume Δθis so great that it may lower the accuracy of the overall alignment inthe E.G.A. system, the wafer holder on the wafer stage ST should berotated in the reverse direction finely by the volume Δθ.

Subsequently, the two shots are again given the sample alignment toverify that the rotation volume Δθ between sufficiently small. Then, thesample alignments are performed for the remaining shots to execute theE.G.A. operation.

In the above-mentioned sample alignments, the wafer alignment sensorshown in FIG. 1 and others is used to pick up the multimark with thewideband illuminating light. Therefore, there is no interferencephenomenon due to the resist layer, and it is possible to carry out astable measurement of the mark positions. In the mark positionmeasurements, at the same time that the amounts of deviation Δx and Δybetween the center Ct of the index marks TL and TR and the center Cl ofthe wafer mark are obtained, the stop coordinating value of the stage STat that the should be read from the interferometers IFX and IFY for thestorage.

As set forth above, in each of the embodiments according to the presentinvention, the descriptions have been made mainly of the utilization ofthe alignment sensor for the mark image detection using the widebandilluminating light in consideration of the influence of the resist layeron the wafer. In the recent years, however, there has been proposed amethod whereby to remove the resist layer only for the wafer markportions. In this case, it is unnecessary to provide a wideband markillumination light, and an alignment sensor using an illuminating lightof single wavelength such as laser light can possibly be employed. Thepresent invention is equally applicable to the case where the waveformof the video signal or photoelectrical signal obtained by the alignmentsensor using the illuminating light of such single wave length isanalyzed. In such a case, if the resist layer for the mark portions hasbeen removed, the waveform becomes a simple waveform such as having itsbottom (or peak) at the mark edge as shown in each of the embodiments,and it is equally possible to deal with the effect of the asymmetry ofthe mark.

I claim:
 1. An apparatus for detecting an alignment mark formed on asubstrate at a predetermined position thereon, the apparatuscomprising:(a) an alignment optical system for forming an image of alocal area of the substrate at a predetermined imaging plane, the sizeof said local area being capable of containing said alignment mark; (b)a photoelectric image detector disposed at said imaging plane forscanning said image of the local area in a predetermined detectingdirection and generating a photoelectric signal with a waveformrepresenting said image of the local area; (c) a waveform memory forperforming digital-sampling of said waveform of the photoelectric signaland storing the sampled digital waveform data; (d) a movable stage formoving the substrate with respect to said alignment optical system so asto shift a position of said local area on the substrate by apredetermined shift amount in said detecting direction, said shiftamount being determined such that the local area before shifting and thelocal area after shifting are partially overlapped with a predeterminedoverlapping width; and (e) a processor for joining digital waveform datastored by said memory before the shifting of the substrate and digitalwaveform data stored by said memory after the shifting of the substrateinto joined successive waveform data and recognizing a waveform portioncorresponding to said alignment mark from said successive waveform data.2. An apparatus according to claim 1, wherein said processor corrects atleast one of the stored digital waveform data in such a way that twodigital waveform data portions corresponding to said overlapping widthhave substantially equal contrast values.
 3. An apparatus according toclaim 2, wherein said processor calculates an averaging level of each ofsaid two digital waveform data portions corresponding to saidoverlapping width.
 4. An apparatus for detecting a position of asubstrate provided with an alignment mark formed thereon, the apparatuscomprising:(a) an alignment optical system for forming an image of saidalignment mark at a predetermined imaging plane and a two-dimensionalimage pick-up device for receiving said image of the alignment mark andgenerating a picture signal which includes a waveform corresponding tosaid alignment mark; (b) an index plate fixedly disposed in an opticalpath of said alignment optical system, having a first region for finealignment and a second region for search alignment and having an indexmark in said first region, said image pick-up device picking up imagesfrom said first and second regions; (c) a movable stage for moving thesubstrate to position said alignment mark at first or second locationswhich are included within an imaging field of said alignment opticalsystem, said first and second locations respectively corresponding tosaid first and second regions of said index plate; and (d) a processorhaving a first mode for processing a waveform of a picture signalgenerated from said image pick-up device by images from the first regionof the index plate at the time of said fine alignment and a second modefor processing a waveform of a picture signal generated from said imagepick-up device by an image from the second region of the index plate atthe time of said search alignment.
 5. An apparatus according to claim 4,wherein said two-dimensional image pick-up device has a horizontalscanning direction on an image receiving surface, and said first andsecond regions of the index plate are arranged successively in adirection normal to said horizontal scanning direction.
 6. An apparatusaccording to claim 4, wherein said alignment optical system includes anintermediate imaging plane formed by an objective lens and a re-imagingoptical lens for forming said imaging plane of said alignment opticalsystem so as to be conjugate with said intermediate imaging plane, andan image receiving surface of said image pick-up device is disposed onsaid imaging plane of said alignment optical system.
 7. An apparatusaccording to claim 6, wherein said index plate comprises a transparentplate disposed on said intermediate imaging plane, said index mark beingformed on a region of said transparent plate which constitutes saidfirst region of the index plate.
 8. An apparatus according to claim 6,wherein said two-dimensional image pick-up device has a horizontalscanning direction on an image receiving surface, and said index markincludes a pair of first index marks separated in a directioncorresponding to said horizontal scanning direction of the image pick-updevice and a pair of second index marks disposed outside said firstindex marks, respectively.
 9. An apparatus according to claim 8, whereina space between said first index marks in said horizontal scanningdirection is larger than a space between each first index mark and arespective second index mark in said horizontal scanning direction. 10.A method for detecting an alignment mark formed on a predeterminedposition of a substrate, comprising the steps of:(a) generating a firstvideo signal corresponding to an image of a first local area of thesubstrate by using a two-dimensional image pick-up system and storingwaveform data of said first video signal; (b) shifting the substraterelative to said image pick-up system in a scanning direction of saidimage pick-up system by a predetermined shift amount; (c) generating asecond video signal corresponding to an image of a second local area ofthe substrate by using said image pick-up system after said shiftingstep and storing waveform data of said second video signal, said secondlocal area being arranged so as to partially overlap said first localarea, said alignment mark being included in at least one of said firstand second local areas; (d) joining said stored waveform data of thefirst and second video signals at waveform portions thereofcorresponding to the overlap of said first and second local areas, andproducing successive waveform data of said first and second videosignals; and (e) searching a waveform portion corresponding to saidalignment mark from said successive waveform data and determining aposition of said alignment mark.
 11. A method according to claim 10,wherein said first local area is determined based on the result of aprealignment of the substrate.
 12. A method according to claim 11,wherein a level of one of said stored waveform data is corrected suchthat contrast values of waveform portions corresponding to said overlapare substantially equal to each other.
 13. A method according to claim12, wherein said shift amount is determined in such a way that a lengthof the overlap in said scanning direction of the image pick-up systemcorresponds with a length of a mark forming area of said alignment markon the substrate in said scanning direction.
 14. A method according toclaim 13, wherein a size of each of said first and second local areas isin accordance with a predetermined portion of an image detecting surfaceof said image pick-up system.
 15. A method according to claim 14,wherein an image of said alignment mark received by said image pick-upsystem is smaller than said predetermined portion of said imagedetecting surface of the image pick-up system.
 16. A method according toclaim 13, wherein said alignment mark comprises at least three markpattern elements which are arranged in said scanning direction of theimage pick-up system and formed within said mark forming area on thesubstrate.
 17. A method according to claim 10, wherein saidtwo-dimensional image pick-up system comprises a CCD camera forgenerating said video signals and an imaging optical system for formingan image of said first or second local area on an image detectingsurface of said CCD camera with a predetermined magnificationsubstantially between 30 and 50 times.
 18. A method for detecting analignment mark formed on a substrate and aligning the substrate inaccordance with the detecting result, the method comprising the stepsof:(a) providing an index mark member, an image detecting system havingan alignment optical system to form an image of said alignment mark on apredetermined final imaging plane and to form an image of an index markof said index mark member in a first region of said final imaging plane,and a two-dimensional image pick-up device disposed substantially atsaid final imaging plane to receive said image of the alignment mark andsaid image of the index mark; (b) detecting the images of said alignmentmark and said index mark simultaneously in said first region of saidfinal imaging plane by said two-dimensional image pick-up device andprocessing a first video signal generated from said two-dimensionalimage pick-up device when performing a fine alignment of the substratebased on the images formed in said first region; (c) detecting the imageof said alignment mark in a second region of said final imaging plane bysaid two-dimensional pick-up device without disturbance of the image ofsaid index mark and processing a second video signal generated from saidtwo-dimensional image pick-up device when performing a search alignmentof the substrate based on the image formed in said second region; and(d) determining a positional deviation of said index mark and saidalignment mark based on said processing of the first video signal whenperforming said fine alignment and determining a position of saidalignment mark based on said processing of the second video signal whenperforming said search alignment.
 19. A method according to claim 18,wherein said alignment optical system includes a first lens system toform an intermediate image of said alignment mark, and a second lenssystem re-imaging said intermediate image of the alignment mark on saidfinal imaging plane, and said image pick-up device includes a CCD camerahaving an image receiving surface disposed on said final imaging plane.20. A method according to clam 19, wherein said index mark is disposedat a plane of said intermediate image of the alignment mark, andarranged in such a way that the image of said index mark is formed at aside portion of the first region of said final imaging plane.
 21. Amethod according to claim 20, wherein said image receiving surface ofthe CCD camera has a horizontal scanning direction and said first andsecond regions of the final imaging plane are disposed successively in adirection perpendicular to said horizontal scanning direction.
 22. Amethod according to claim 18, further comprising a step of positioningthe substrate when performing said search alignment in such a way thatthe image of said alignment mark is positioned in the second region ofsaid final imaging plane.
 23. A method according to claim 22, whereinsaid fine alignment is performed after carrying out said searchalignment based on said position of the alignment mark determined bysaid search alignment.